1. Field of the Invention
The present invention relates to a cylinder head.
2. Description of the Related Art
Known in the art is a cylinder head where exhaust ports of adjoining cylinders are gathered together at an exhaust merging portion inside the cylinder head, a partition wall separating the exhaust ports of these adjoining cylinders extends from between these adjoining cylinders to the exhaust merging portion, a head bolt insertion hole is formed inside the partition wall, and an oil passage is formed inside the partition wall between the front end of the partition wall facing the exhaust merging portion and the head bolt insertion hole (see Japanese Patent No. 3605521). In an internal combustion engine provided with this cylinder head, the temperature of the exhaust merging portion where the exhaust gas is collected from the cylinders becomes particularly high, so the temperature near the front end of the partition wall facing this exhaust merging portion becomes the highest. Therefore, in this internal combustion engine, the temperature of the inner circumference of the oil passage formed inside the partition wall becomes high, so at the time of a low temperature, the oil flowing through the oil passage is quickly made to rise in temperature.
On the other hand, in an internal combustion engine provided with this cylinder head, there is a danger of the oil overheating. Therefore, there is known a cylinder head forming a cooling water passage adjoining the oil passage inside the partition wall (see Japanese Patent Publication (A) No. 2002-70641). In an internal combustion engine provided with this cylinder head, the oil passage is cooled by the cooling water flowing inside the cooling water passage, so the oil can be prevented from overheating.
In this way, when using the cylinder head described in Japanese Patent No. 3605521, the problem arises that the oil flowing inside the oil passage is overheated and the oil ends up being baked on the inner circumference of the oil passage. Further, in the case of using the cylinder head described in Japanese Patent Publication (A) No. 2002-70641, the temperatures of the inner circumference of the oil passage positioned at the side of the cooling water passage fall, but no means is provided for suppressing the transfer of heat between the inner circumference of the oil passage positioned at the opposite side to the cooling water passages and the front end of the partition wall, so the problem arises that the inner circumference of the oil passage positioned at the opposite side to the cooling water passage is overheated and therefore the oil ends up being baked on the inner circumference of the oil passage.
Further, in the cylinder heads described in Japanese Patent No. 3605521 and Japanese Patent Publication (A) No. 2002-70641, no particular care is given to suppressing the transfer of heat to the head bolt insertion hole boss formed inside the partition wall, so the temperatures of the head bolt insertion hole boss formed inside the partition wall becomes considerably higher than other head bolt insertion hole bosses. As a result, the head bolt insertion hole boss formed inside the partition wall expands by heat greater than the other head bolt insertion hole bosses, so great compressive stress is formed in the head bolt insertion hole boss formed inside the partition wall and therefore there is the problem that the head bolt insertion hole boss formed inside the partition wall drops in durability.